Mechanical properties of NiAl-Mo composites produced by specially controlled directional solidification
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Mechanical properties of NiAl-Mo composites produced by specially controlled directional solidification G. Zhang1, L. Hu1*, W. Hu1, G. Gottstein1, S. Bogner2, A. Bührig-Polaczek2 1
Institute for Physical Metallurgy and Metal Physics, RWTH Aachen University, Kopernikusstraße 14, D-52074 Aachen, Germany 2 Foundry Institute, RWTH Aachen University, Intzestraße 5, 52056 Aachen, Germany * Correspondent author. Tel.: +49 (0) 241 80 20 25 7; fax: +49 (0) 241 80 22 30 1. E-mail address: [email protected] ABSTRACT Mo fiber reinforced NiAl in-situ composites with a nominal composition Ni-43.8Al9.5Mo (at.%) were produced by specially controlled directional solidification (DS) using a laboratory-scale Bridgman furnace equipped with a liquid metal cooling (LMC) device. In these composites, single crystalline Mo fibers were precipitated out through eutectic reaction and aligned parallel to the growth direction of the ingot. Mechanical properties, i.e. the creep resistance at high temperatures (HT, between 900 °C and 1200 °C) and the fracture toughness at room temperature (RT) of in-situ NiAl-Mo composites, were characterized by tensile creep (along the growth direction) and flexure (four-point bending, vertical to the growth direction) tests, respectively. In the current study, a steady creep rate of 10-6s-1 at 1100 °C under an initial applied tensile stress of 150MPa was measured. The flexure tests sustained a fracture toughness of 14.5 MPa·m1/2 at room temperature. Compared to binary NiAl and other NiAl alloys, these properties showed a remarkably improvement in creep resistance at HT and fracture toughness at RT that makes this composite a potential candidate material for structural application at the temperatures above 1000 °C. The mechanisms responsible for the improvement of the mechanical properties in NiAl-Mo in-situ composites were discussed based on the investigation results. INTRODUCTION In-situ NiAl composites reinforced by refractory metallic phase like Mo through eutectic reaction has been considered to be an effective way to improve both the fracture toughness at RT and the creep resistance at HT of binary NiAl. Some substantial progresses have been achieved since the early 1990s. Joslin et al. [1] measured creep strength of 80 MPa at 1300K and a strain rate of 10-6s-1 on directionally solidified (DS) NiAl-9Mo, compared to the creep strength of 20 MPa at 1300K and 10-6s-1 on [001] NiAl. Misra and Heredia reported a RT fracture toughness of around 15 MPam1/2 on DS NiAl-9Mo [2, 3], compared to the RT fracture toughness of [001] NiAl of about 8 MPam1/2. However, due to the low temperature gradient (3~5 K/mm) of the Bridgeman furnace they used for directional solidification of NiAl-9Mo, the microstructure of as-produced materials was mainly comprised of cellular structures with irregularly aligned Mo fibers. In 2005, Bei et al. has successfully produced a unidirectional Mo fiber reinforced NiAl composite with regular alignment of fibers using an optical floating zone furnace, in which an ultimate tensile streng
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